Ingot mold and ingot



lub' 18, l939 E. GATHMANN INGOT MOLD AND INGOT Filed June l, 1938 4f :I LJ/I LK,

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Fm/7 Gahmann Patented July 18, 1939 INGOT MOLD AND INGOT Emil Gathmann, Catonsville, Ma., assignmto Gathmann Research Incorporated, Catonsville, Md., a corporation of Maryland Application, June 1, ms, Serial No. 211,285

Claims.

This invention relates to ingot molds and ingots, and more particularly to the vertical contours of big-end-up mold chambers and ingots.

This application is a continuation in' part of 5 my application, Serial No. 159,405, filed August As is now well known to those familiarwith the art, molds generally designated as big-endup molds possess certain desirable characteristics and are the only type in which ingots of interior soundness can be produced commercially.

There are, of course, many factors influencing mold and ingot design; and the perfect ingot, adjudged from all practical standpoints, never has been and probably never will be produced commercially because of the incompatability of design factors. For example, in theory a very large degree of upward and outward taper in the mold chamber is metallurgically desirable, as a congg tinued increase in cross section from the bottom towards the Atop of the chamber is necessary to effect progressive upward solidiflcation of the liquid ingot metal, an essential to soundness. A

, large degree of taper also shortens the time period of solidification due to the closer contact of the mold matrix with the forming ingot; and, as segregation of carbon and other elements in the steel is directly proportional to the time of solidificatlon, it heretofore quite naturally has been the belief that a large continuous taper materially reduces the degree of segregation. I have found, however, that an excess of continuous taper will defeat its own purpose by preventing the ingot from settling into the mold as the metal shrinks and solidies. In a mold having a large continuous taper from bottomto top, the partially solidified ingot is caught by the walls of the mold at the upper portion of the chamber and is hung there, forcing the ingot to shrink upwardly upon itself and causing an air gap to form between the ingot surface and the mold chamber Walls in the bottom and intermediate portions. The formation of such an air gap prevents the progressively upward extraction of heat from the ingot by the mold, retarding solldication and increasing segregaton. The weight of the ingot is usually more than the strength of the initially-solidified surfaces of the ingot can bear, and the skin and subskin structure are ruptured horizontally, producing the well-known hanging tears in the ingot below the shrinkhead portion. Furthermore, a large continuous taper is objectionable in mill practice because the ingot must be given an excessive number of initial blooming passes in reducing it to the required uniform cross section if the outer structure is not to be torn or lapped by the rolls.

It therefore is apparent that in order to improve upon prior and existing mold and ingot practice, a mold must be constructed with re- 5 gard not to one factor but to various considerations, such as interior and surface ingot soundness, amount of top and bottom crop loss, rolling properties as affecting both-the way the ingot metal is worked in the rolls and the cost per ton 10 of product of reducing the ingot in the rolls to the required section, mold life, and so forth.

The object of this invention is to provide an improved mold and ingot of the big-end-up type in which the vertical sectional contour of the l5 walls defining the mold matrix and surface of the ingot are so balanced with respect to other factors, including the several factors referred to above, as to promote soundness and reduced segregation of the metal throughout the interior 20 of a well-degasifled ingot, to reduce surface defects of the ingot, to reduce crop loss, to facilitate initial reduction of the ingot to the desired shape and to prolong mold life.

A further object is to provide an improve- 25 ment in the life of vertically corrugated mold chambers.

Other objects will become apparent from a reading of the following description, the appended claims, and the accompanying drawing, in which: 30

Figure 1 isa central vertical sectional view of a mold embodying the invention, the section being taken on the line I-I of Figure 2;

Figure 2 is a top horizontal plan view of the mold shown in Figure 1 with a shrinkhead cas- 35 ing removed;

Figure 3 is a vertical side elevation of a steel ingot embodying the invention; and

Figure 4 is a cross section on line 4 4 of Figure 3 showing graphically the types of crystallization 40 in the ingot shown in Figure 3.

My invention, which, as stated above, relates to the vertical contours of big-end-up mold chamber walls and ingot surfaces, may be embodied to particular advantage in molds and in- 45 gots having the corrugated contours disclosed in my U. S. Patent No. 2,092,551, issued September 7, 1937, but also may be embodied in molds and ingots having various other cross sectional contours including, for example, slab type molds and 50 ingots as well as molds and ingots of the kind disclosed herein. An at present preferred cross sectional contour is shown in Figures 2 and 4 of the drawing.

A number of theories regarding the formation of an ingot are subscribed to by metallurgists. I have found that usually an ingot consists of three crystallization zones. Figure 4 shows an ingot I in which the three zones of crystallization are denoted: 4

a. A zone or layer of chill minute equiaxed crystals; b. A zone or layer of dendritic or columnar crystals; c. A central zone of larger equiaxed crystals.

The physical properties of the chill crystals o as well as the equiaxed crystals c usually are much better than those of the dendritic or columnar crystals b. The initial or chill crystals a form a tough protective sheathing for the ingot, tending to resist the formation of surface cracks during solidiflcation and contraction, and it therefore is desirable that the ingot be cooled in such a manner as to produce a relatively thick layer of chill crystals. The extreme outer or surface portion of the chilled crystal layer forms imrnediately upon contact of the molten metal with the mold walls; and, in order to cause the chill effect to extend inwardly from the extreme surface of the ingot, it isl necessary to maintain the bottom and side surfaces of the ingot in contact with the mold walls for a substantial period. Soon after the pouring of an ingot has been completed. the mold begins to expand, increasing in chamber cross section and length, and the ingot beginsto contract, decreasing in cross section and length. Hence, if a mold has substantially vertical walls, the walls and ingot will beconie separated soon after pouring, causing the formation of an insulating air space between the mold walls and ingot and removing the chilling effect of the mold walls. The formation of this air space may be delayed materially, as is now known, by providing mold walls which taper upwardly and outwardly (big-end-up), such an arrangement resulting in substantially longer contact of the mold walls and ingot surfacemetal during successive stages of the cooling of the ingot. Thus, by providing walls tapered as stated above, it is possible to obtain a decidedly thicker initial chill crystallization a than when using big-end-down or parallel mold walls. I have found that, generally stated, the thickness of the chill crystallization zone is greater when the degree of big-end-up taper is greater; that is, a mold matrix having a high degree of such taper will produce a chill crystal zone thicker than a mold matrix having a less degree of taper. Hence, insofar as the formation of the desired chill crystals a is alone concerned, it is desirable to employ a rather marked degree of big-end-up taper in the mold matrix walls.

On the other hand, if the mold chamber walls are highly tapered uniformly upwardly and outwardly from bottom to top, certain difficulties are encountered. One of such' diiiiculties, as previously stated, results from a tendency of the ingots to hang in the mold walls in the top of the mold chamber just below the shrinkhead, so that the ingot is not free to descend during solidication to maintain contact of the body and lower portion of the ingot surface with the body and bottom mold chamber walls, which contact is essential for` progressive, upward solidifcation. In my United States Patents 1,643,241 of September 20, 1927, and 1,819,705 of August 18, '1931, I have disclosed ingot molds in which the chamber is tapered upwardly and outwardly to a greater degree in its lower portion than in its extreme upper portion. This construction eliminates some of the difdculty previously encountered because of hanging of the upper part of the ingot surfaces in the mold matrix, and has been found advantageous in the making of millions of tons of commercial ingots. The constructions disclosed in the patents referred to furthermore have the advantage that the total difference in cross sectional dimension between the small end of the ingot and the large end of the ingot required to insure progressive solidification may be reduced somewhat, and in addition uniform reduction of the ingot surface in the rolling mills is considerably facilitated.

I now have found, through the production and examination of many ingots, that a metallurgically more sound and a more easily rollable ingot can be produced in a mold in which the extreme lower portions of the side walls of the chamber are tapered to a considerably greater degree than the intermediate body side wall portions adjoining and extending above the extreme lower side wall portions. Results observed in commercial production have established that the lower side wall portions, comprising from about 20% to 50%, preferably about one-third, of the total side wall height of the ingot body, should be tapered from two to twenty times as much as the taper of the upper and adjoining tapered walls.- The amount of taper per inch of vertical height in the lower side walls should be increased as the cross section of the mold chamber increases and should extend at least 2 but not more than 20 outwardly and upwardly in relation to the vertical axis of the mold, but should not exceed about' twenty times the taper per inch of height of the upper adjoining tapered side walls of the chamber, regardless of the cross section.

I have found that in mold chambers of about twenty-inch maximum cross section, the bottom side wall portions should be tapered about four times as much per inch of height as the main or intermediate side wall portions. The result of this differential taper is to produce an initial chill zone a which is thicker in the bottom side surface portions of the ingot than in the upper adjoining side surface portions, and,` furthermore, considerably longer and more uniform contact of all of the side surface portions of the ingot with the mold chamber walls is assured. Because of this differential taper, the isothermal layers of chilling and crystallization a at the lower sides of the ingot for a distance of from 20% to 50% of the ingot height will be directed upwardly and inwardly to a greater degree than in ingots made in accordance with previous practice in which the bottom portions of the ingot side walls are tapered to substantially the same extent per inch as the adjoining upper portions. The effect is to produce a more uniform progressive solidification from bottom to top of the ingot than has heretofore been possible. To further promote the desired progressive solidication, the mold 4chamber walls preferably should diminish in thickness progressively from bottomv to top.

Figure 1 shows how the invention may be embodied in a mold generally designated M comprising opposed side walls as a whole designated i and a bottom or bottom wall generally designated 2. Lifting and handling trunnions L of a known kind may be provided. The mold is shown as being provided with a hot top or shrinkhead casing H, which may be of known construction. Both the lower portions 3 of th'e mold matrix side walls (from A to B), and the main or body portions ,4 (from B to C) are substantially straight usA ' the mold chamber.

and are tapered upwardly and outwardly from the longitudinal axis Y of the mold chamber; but the lower portions"3 are tapered upwardly and outwardly to a far greater degree per foot of height than are the main or body portions 4. In a mold chamber of twenty-inch maximum c'ross sectional dimension, the taper of the body portion 4 should preferably be about half an inch per foot, and the lower wall portions 3 preferably should taper about two inches per foot. Thus, the lower wall portions 3 are tapered about four times as much per foot as the adjoining upper portions 4. It will be understood, however, that the relative amount of taper should vary in dependence upon the size of the mold chamber and the specication vof the steel forming the ingot. Likewise, the height of the more greatly tapered lower wall portions 3 varies in dependence upon the size of In a mold of twenty-inch chamber cross section, the lower portion 3 preferably is about one-third of the total height of the mold chamber. The relative amounts of taper and the relative height of the more greatly tapered portion 3 may also vary'considerably, independence upon the horizontal cross section of moldl chamber employed, but the lower tapered portion 3 should preferably in any case be between about 20% and 50% of the total chamber length of the metallic mold exclusive of the shrinkhead casing H, ,and'it should be tapered from about two to twenty times as much per inch of height as the adjoining upper side walls 4.

The dotted lines X and the dotted lines W show clearly the great difference in the amount of taper of the wall portions 3 and the wall portions 4.

kIt is usually advantageous to employ an extreme upper wall portion 5 of substantially parallel contour, shown extended at Z. Depending upon the size and cross section of the ingot, this section 5 may be of varying length, but should not exceed in length the inside diameter of the hot topor shrinkhead casing H, and in .vertical height should be from about 5% to' about 20% of the total mold chamber height.

It is desirable to provide smoothly rounded illlets F between the straight mold wall portions 3 and 4, and 4 and 5 of the mold chamber. The provision of these smooth fillets prevents the formation of horizontal surface cracks in the mold chamber, and, furthermore, prevents hanging of the ingot, which might occur if a sharp angle were left between the wall portions 3 and 4 and 4 and 5. i

Any type of longitudinally corrugated or planesided cross section of mold chamber may be employed. However, when corrugations are used, I prefer to have the inwardly and outwardly extending corrugations R and S of the mold chamber walls substantially uniform in depth from the top of the mold chamber portion 5 to the top of the mold chamber portion 4, and from the top of the mold chamber portion 4 to the upper part of the more highly tapered lower mold chamber portion 3. 'Ihe corrugations being of a plane vertical contour will thus merge or vanish in the upper third or half of the highly tapered section A 3 of the mold chamber, substantially as shown in the drawing at V in Figure 1. This type of corrugation provides a lower mold chamber of substantially rectangular contour with four preferably rounded corners e. Such construction increases the mold life materially, as the lower part 3 of the mold chamber walls is subject to the washing and erosive action of the molten ingot .differentially tapered portions or sections.

metal as it is teemed into the mold chamber, and the corrugations, if extended to the bottom 2 of the chamber, are frequently burned and washed away to such an extent that after relatively few ingots have been produced in the mold, stripping of the ingots from the mold is at times rendered quite difficult, thus delaying stripping operations as well as shortening the mold life. Since there are no projecting corrugations R in the lower side walls of the mold chamber 3, there' is less danger of cutting the walls of the chamber due to this surging action of .the steel as it is being teemed and danger of the ingots sticking to the lower part of the mold chamber walls 3 is greatly reduced and the mold life lengthened.

The contour of the mold bottom wall 2 is extremely important, and the contour disclosed herein forms an important part of my invention. I have found that by far the best results are obtained when the main or central part of the bottom wall 2a is of substantially flattish or slightly dished contour in vertical cross section and the marginal concave portion 2b is struck on a short radius.v The concave portion 2b intervenes between and merges with the main bottom wall portion 2a and the lower side wall portions 3. In the preferred form, disclosed in Figure 1 for the purposes of illustration, the bottom wall 2-2a2b is integral with the vertically extending walls 3, 4, and 5. and is apertured for the reception of a removable plug P. The plug may be a closure plug of any suitable kind or, in cases in which the practice of any particular ingot casting plantrequires bottom pouring, the moldbottom may be apertured to form a gate for introducing molten steel through the bottom of the mold in the usual manner of bottom teeming. The main bottom wall portion 2a, when dished, should be struck on a relatively long radius K, and the marginal Wall portion 2b should be struck on a relatively short raidus r, the length of which latter radius should be not lesslthan about 5% and not more than about 20% of the maximum cross section measured between opposite side walls having the The use of relatively small radii for the marginal portion 2b is important because it has been found that large radii are more apt to produceV subcutaneous cleavage planes and surface cracks in the ingot bottom than are the relatively smaller radii. The use of a relatively long radius K for the bottom wall proper 2a produces a relatively shallow dished contour, rather than a deeply rounded contour. 'Ihe relatively shallow dished contour 2a is much to be preferred to a deeply hollowed-out bottom wall because it results in less butt crop when the ingot is reduced to a bloom or slab.

As is apparent, an ingot cast in a mold embodying my invention will have a contour corresponding to the mold chamber contour already described. Briefly, however, it may be stated that the ingot I shown in Figure 3 has side surfaces l formed with corrugations R'-S corresponding to the corrugations R-S of the mold chamber walls, and a bottom surface 2-2a'2b. 'Ihe lower portions 3 of the side surfaces are tapered to a greater degree than are the intermediate si'de surfaces 4' above and adjoining the lower surfaces 3'.

lower surface portions 3 comprises from about 20% to about 50% of the total vertical length I', .and preferably the lower side surface portions 3' are tapered from about two to about twenty times more than the intermediate side surface Preferably the vertical length of thel portions 4' above and adjoining the surface portions 3'. The extreme upper surface portions 5 of the ingot may be tapered to an even less degree than the intermediate surface portions 4' or may be substantially straight or parallel with the longitudinal axis Y of the ingot. The lines X', W and Z indicate graphically the difference between the upward and outward tapers of the lower side surface portions 3' and the surface portions 4 and 5' thereabove.

The shrinkhead portion H of the ingot consists primarily of dendritic crystals similar to the crystals in thezone b of the ingot proper, although the crystals in the shrinkhead are usually somewhat larger than those in the zone b, as shown in Figure 4. because the metal has cooled and solidified more slowly than in the .body of the ingot.

Because of the greatly increased upward and outward taper in the botom portions 3 of the side walls of the mold chamber, these portions of the mold chamberfwalls are rendered somewhat more heavy than the upper portions and as at least the lowermost portion of said side walls preferably is not corrugated, some cutting of these lower side walls by the erosive action of the stream of molten metal poured into the mold may take place without deleterious results. Thus, even though some of the lower chamber side wall portions 3 are cut away by the molten stream, the increased taper is such that even considerable cutting will not result in serious sticking ofthe ingot within the mold chamber portion 3, and thus will not interfere with the stripping of the ingot after it has solidified.

In the rolling of' an ingot I cast in a mold having the contour described, there will be less unworked section andY hence less bottom crop than when rolling ingots produced in molds previously known. I have found that the bottom crop lost with ingots cast in my improved mold chambers will be less than bottom crops from ingots heretofore produced, by as much as 25% to 50%. Furthermore, the improved new bottom contour, particularly as regards the fiattish or slightly dished main bottom portion and the concave marginal chamber portion, will result in the production of lan ingot .base which will more readily stand in vertical position in the soaking pit and which will sink less deeply into the layer of coke forming the floor of the soaking or reheating pit. Thus, a smaller part of the side walls of the ingot will be decarburized or cut by the action of the heated coke on the ingot metal.

55 As is now well known to those familiar with the art, the highest yield of sound homogeneous steel is obtained from ingots cast of fully deoxidized or degasified steel in molds so designed that from 80% to 95% of the ingot is formed and completely solidified in a heat-absorbing metallic mold having a bigendup chamber surmounted by a heat insulating hot top or shrinkhead casing designed to contain from 5% to 20% of the total molten ingot volume. In using molds having the differential chamber tapers according to my present invention, the best results are usually obtained with the following practice. In top-teemin'g, I prefer to use a method that includes relatively slow pouring until a pool extending to approximately the junction of the mold chamber walls 3 with the mold walls 4 is formed. By this method, the surging and splashing and cutting of the lower mold chamber side walls and the mold chamber bottom 'are Ilargely eliminated.

Preferably, the stopper of the pouring nozzle should be opened only about one-fifth to one- 4half of its full capacity until such a pool has been formed. Thereafter the stopper may be opened wide, since the danger of cutting or erosion of thevmold walls ceases after the pool has been formed, providing, of course, that the ladle nozzle has been reasonably well centered. The full pouring rate should bereduced as soon as the metal has risen to the top of the mold chamber proper and has commenced to fill the shrinkhead casing. The casing chamber itself should preferably be teemed at from one-tenth to onehalf of the rate of a fully-opened stopper.

The method of producing an ingot of welldeoxidized steel in accordance with my present invention comprises simultaneously cooling the ingot in four vertical zones at different rates and chill cooling the body of the ingot, exclusive of the shrinkhead portion, at three different rates. Thus, the bottom 2' and the lower side surface portions 2b and 3', comprising from about 20% to about 50% of the ingot length are chill-cooled at a. relatively very rapid mean rate, the intermediate side surface portions l' are chill-cooled at a relatively slower mean rate, the extreme upper side surface portions 5', comprising from about 5% to about 20% of the ingot length, are chill-cooled at a relatively still lower rate and the shrinkhead portion H is cooled at the lowest rate. The rate of cooling the lower side portions 3' decreases progressively throughout these portions from their bottoms to their tops, and the rate of cooling the intermediate side portions 4' decreases progressively throughout these portions 4' from their bottoms to their tops less rapidly than the decrease of cooling rate from bottom to top of the lower side surface portions 3'. The rate of cooling is determined on the basis of the lapse of time between the completion of teeming and the solidication of the metal.

By virtue of my novel vmold chamber and ingot surface contour, comprising the differentially tapered side walls and the fiattish or dished bottom, it is possible to obtain ingots of materially improved interior and surface soundness, to prolong mold life, to reduce crop loss, and to facilitate reduction in rolling or forging. The total taper or difference in cross section at the top of the mold chamber and the bottom of the chamber in a mold of twenty-inch maximum cross section would be approximately only four to five inches, thereby making for good rolling properties. Nevertheless, the lower part of the chamber 3, and correspondingly the ingot at 3', has four times as much taper per inch of vertical height as the adjoining upper tapered portion, so that the initial chill solidification exltends into the ingot more deeply at the bottom and the time period of solidifcation of the entire ingot is decreased. The quicker cooling of the lower portion of the ingot both on its sides and its bottom enhances the progressive cooling and solidication of the ingot from bottom to top of the mold, thereby producing an ingot body which is physically sound throughout and requiring a minimum top discard to eliminate the physically defective and positively segregated portion and a minimum bottom discard to square up the bloom and eliminate the negatively segregated portion.

The life of big-end-up molds usually is determined by the amount of erosion resulting Ifrom the cutting action of the stream of molten steel impacting against the bottom and the lower side walls of the mold chamber. When using molds embodying my improved mold matrix contour, either with or without corrugations in the upper scribed in its at present preferred form, but it will be understood that various changes may be made without departing from the invention as defined in the' claims.

I claim:

1. A metallic ingot mold having a big-end-up ingot-forming chamber defined by side Walls and a bottom wall, at least two opposed side walls being in three sections from bottom to top and each of said sections being substantially straight in vertical contour, the lowermost opposed side wall sections comprising from 20% to 50% of the total chamber side wall length being tapered upwardly and outwardly to a materially greater degree than the adjoining intermediate opposed side wall sections, andsaid opposed intermediate-sec-v tions ,being tapered upwardly and outwardly more than the opposed extrei'n upper side wall sections. and the-mold walls progressively decreasing in thickness from bottom upwardly in said lower side wall sections and in said side wall sec'- tions above and adjoining said lower side wall sections.

2. A metallic ingot mold having a big-end-up ingot-forming chamber defined by side walls havin g portions which are corrugated in horizontal cross section, the lower portions of at least two opposed side walls, comprising between from about 20% to about 50% of the total side wall length, being substantially straight in vertical section and having a substantially greater degree of taper than the opposed side wall portions above andadjoining said opposed lower side wall portions, the corrugations extending downwardly l into said lower side wall portions but merging tal cross section and a mainly attish bottom surface, the lower portions of at least two opposed' side surfaces, comprising between from about 20% to about 50% of the total side surface length, being substantially straight in vertical section and having between two and twenty timesA the degree of taper of the opposed side surface portions above and adjoining said opposed lower side surface portions, and said bottom'surface including a convex marginal portion merging with said opposed lower side surface portions, said convex marginal portion being struck on a radius not less ,than 5% and not more than 20% of the maximum chamber cross section measured between said opposed side surfaces, the corrugations extending downwardly into said lowerside surface portions but merging therewith short of the bottom.

4. In a metallic ingot mold, a vertically-extending big-endfup ingot forming chamber dened by upwardly and outwardly tapered side tapered vertical contour, the lower substantially straight outwardly tapered portions of at least two opposed side walls, comprising between 20% and 50% of the total side wall length of the chamber, having a materially greater degree of taper than the opposed substantially straight outwardly tapered side wall portions above and adjoining said opposed lower side wall portions, and the mold walls progressively decreasing in thickness from bottom upwardly in said lower side wall portions and in said side wall portions above 'and adjoining said lower side wall portions.

5. In a metallic ingot mold, a vertically extending big-end-up ingot forming chamber defined by upwardly and outwardly tapered side wall portions of substantially straight outwardly tapered vertical contour, the lower substantially straight outwardly tapered portions ofat least two opposed side walls, comprising approximately one-third ofthe total side wall length of the chamber, having a materially greater degree of taper than the opposed substantially straight, outwardly tapered side wall portions above and adjoining said opposed lower side wall portions,

wall portions of substantially straight outwardly and the mold walls progressively decreasing in.

thickness from bottom upwardly in said lower side wall portions and in said side wall portions above and adjoining said lower side wall portions.

EMIL GATHIANN. 

